1. Field of the Invention
The present invention relates to a power supply apparatus applied to a self-excited switching power supply that converts a high DC voltage obtained by rectifying and smoothing an AC voltage from a commercial power supply into a predetermined low voltage of between a few volts and several tens of volts that is required by an appliance. The present invention also relates to a power supply control method for such power supply apparatus. In particular, the present invention relates to a power supply apparatus applied to a self-excited switching power supply unit that is used in an appliance where there is a clear difference in magnitude of load applied between an operating state and a standby state of the appliance and where there is a marked difference in power required in these two states, and a power supply control method for such power supply apparatus.
2. Description of the Related Art
In general, a self-excited switching power supply unit is designed so as to exhibit the maximum efficiency in a state where the maximum load that is required of an appliance in which the self-excited switching power supply unit is installed is being applied to the appliance. This is essentially required from the viewpoint of thermal design. However, when the load decreases, the efficiency of the self-excited switching power supply unit falls. Such fall in efficiency is increasingly apparent as the load decreases. Accordingly, if the appliance in which the self-excited switching power supply unit is installed is not always in an operating state and can also enter a standby state where the load is extremely low compared to the load in the operating state, the efficiency of the self-excited switching power supply unit is extremely poor.
In order to solve the above problem, Japanese Laid-Open Patent Publication (Kokai) No. 11-332229 has proposed a method for raising the power supply efficiency when an appliance is in the standby mode. In this method (hereinafter referred to as the “the first prior art”, when the appliance is in the standby mode, a reference voltage of an error detecting circuit in a self-excited switching power supply unit is intermittently changed to the ground (GND) level, or a voltage that is higher than the target voltage is intermittently added to the detected voltage so that the self-excited switching power supply unit is intermittently placed in the operating state (hereinafter this will be referred to as “the intermittent operation”).
Japanese Laid-Open Patent Publication (Kokai) No. 2000-156977 has proposed a method of having a self-excited switching power supply unit operate intermittently to raise the power supply efficiency (hereinafter the “the second prior art”). This is achieved by providing a means for transmitting an oscillation stopping signal from the secondary side of the self-excited switching power supply unit to the primary side when the load is light or by switching between two detected voltages that are used for control in the self-excited switching power supply unit.
Further, Japanese Laid-Open Patent Publication (Kokai) No. 2001-190066, has introduced as the “Prior Art” an example where the power supply efficiency is raised while an appliance in which a self-excited switching power supply unit is installed is in the standby state. In this example (hereinafter the “the third prior art”), the self-excited switching power supply unit is operated intermittently by forcibly having the output of an error detection circuit intermittently change to the GND level such that the self-excited switching power supply unit stops when the output of the error detection circuit has been set at the GND level and the self-excited switching power supply unit operates when the output of the error detection circuit has not been set at the GND level.
However, the following problems occur with the conventional intermittent operation control methods for a self-excited switching power supply unit described above.
(1) The First Prior Art
(i) When the error detection circuit of the self-excited switching power supply unit includes an element (generally called a “shunt regulator”) that is internally provided with a reference voltage, the reference voltage cannot be dropped to the GND level. This means that the circuit configuration cannot be freely selected.
(ii) When the self-excited switching power supply unit has only one type of output voltage, a complicated construction is required to raise the detected voltage, so that reductions in cost cannot be made.
(iii) A phase compensation circuit composed of a capacitor and a resistor which is essentially provided in the error detection circuit, acts as an integration circuit, so that during intermittent operation, there always occurs an unintended drop in the output voltage. Also, when the load becomes large during intermittent operation, the influence of this phase compensation circuit causes an abnormal drop in the output voltage.
(iv) Due to the phenomenon described above in (iii), there is the risk of the output voltage of the power supply unit becoming unstable when the appliance shifts from the operating state to the standby state and from the standby state to the operating state.
(2) The Second Prior Art
(i) According to a first embodiment described in the specification of this prior art, an optocoupler is required for transmitting the oscillation stopping signal from the secondary side to the primary side. The addition of an optocoupler that has a high unit price means that reductions in cost cannot be made.
(ii) In both the first embodiment and a second embodiment, the oscillation stops only when the oscillation stopping signal is TRUE. Oscillation is performed when the oscillation stopping signal is FALSE. This means that it is necessary to manage the periods during which the oscillation stopping signal is set at TRUE and FALSE, which makes the control complicated.
(3) The Third Prior Art
(i) The operation of the self-excited switching power supply unit stops only when the output of the error detection circuit has been set at the GND level, so that if the period during which the output of the error detection circuit is set at the GND level is not set correctly, there will be no gain in efficiency and there is the risk of an abnormal drop in the output voltage.
(ii) To solve the above problem, in reality it is necessary to control the periods during which the output of the error detection circuit is set at the GND level and the intervals between such periods, which means that reductions in cost cannot be made.